Epoxy resin compositions are often used as encapsulants (or molding compounds for semiconductor devices) for advanced thin packages. There is the tendency of loading epoxy resins with large amounts of submicron fillers having a mean particle size of less than 1 .mu.m, desirably less than 0.5 .mu.m for achieving both improved loading and minimized moisture pick-up. The fine filler as a whole has a very large specific surface area as compared with conventional fillers. The interfacial wetting between the resin and the filler is thus substantially exacerbated. This results in epoxy resin compositions having an extremely increased viscosity which makes it difficult to mold the compositions.
For the purpose of providing an epoxy resin composition having an increased filler loading and a reduced viscosity, we found that a filler loading of about 80 to 85% by weight and a viscosity of about 100 to 300 poise at 175.degree. C. are achievable by blending a filler containing about 5 to 15% by weight of a spherical filler fraction having a particle size of about 0.5 .mu.m. This type of filler has already been used in practice. On the other hand, the current trend is toward increasing the loading of filler for minimizing water absorption for improving solder reflow resistance. Particularly when a filler loading of more than 85% by weight is desired, conventionally used spherical fillers lead to a very high viscosity in a low shear region to cause die pad deformation, wire deformation and even line breakage.
When epoxy resin compositions loaded with large amounts of inorganic fillers and having a melt viscosity of at most 150 poise, especially at most 100 poise at 175.degree. C. are molded using matrix frames, it is very difficult to control the flow of the resin. There arise the problems of non-uniform filling of first and second cavities with the resin and defects like voids and wire deformations. It is desired to overcome these problems.